The invention relates to a method of making a fillet-seam closure by ultrasonic welding on a folding carton, particularly a peaked-top carton fabricated from boxboard coated with a thermoplastic resin.
In a prior art method of this type, the fillet-seam areas to be welded together after the peaked top of the folding carton has been folded are placed between a so-called anvil and a sonotrode in which vibrations are induced ultrasonically. Through the excitation of the sonotrode with ultrasonic energy, the synthetic-resin layer disposed on the facing sides of the areas to be welded together is softened and fused.
Practical experience has shown that a fillet-seam weld produced ultrasonically cannot be relied on to provide a tight seal on the folding carton, especially when the carton also has an inner aluminum-foil liner. The reason for this, according to U.S. Pat. No. 3,910,014 is that there will be craterlike gaps in the seam weld.